Solid state radio frequency (RF) switches are important components of Radar transmit/receive (T/R) modules, satellite communication systems, Joint Tactical Radio Systems (JTRS), and the like. A promising RF switch technology uses Heterostructure Field Effect Transistors (HFETs). Recently, high power switches made of AlGaN/GaN HFETs demonstrated superior performance over other RF switching devices in terms of maximum power density, bandwidth, operating temperature, and breakdown voltage.
Many applications, including JTRS and low-noise receivers, require RF switches with a very low insertion loss, e.g., typically below 0.1 dB. A low loss switch dissipates little RF power. As a result, it can be fabricated over a low cost substrate, such as sapphire. Low insertion loss in an HFET is due to a high channel conductance of the device, which is proportional to a total length of the device periphery. Exceptionally high 2D electron gas densities at the AlGaN/GaN interface make a group III-Nitride HFET with a total periphery of two to five mm an ideal candidate for RF switching applications.
The feasibility of high-power broad-band monolithically integrated group III-Nitride HFET RF switches has been demonstrated. Large signal analysis and experimental data for a large periphery group III-Nitride switch indicate that the switch can achieve switching powers exceeding +40 to +50 dBm. However, at frequencies corresponding to the RF frequency band, the OFF state isolation achieved by such a switch is limited by its internal parasitic capacitance, which is also proportional to the total length of the device periphery.
In contrast to Micro Electro-Mechanical Systems (MEMS) technology, high performance RF switching implemented using semiconductor devices, including group III-Nitride HFETs and Metal-Oxide-Semiconductor HFETs (MOSHFETs), can only be achieved at relatively low operating frequencies, e.g., below approximately 4-5 GHz, and/or in single channel applications. A significant performance limitation is due to the inherent semiconductor device capacitance in the OFF (high impedance) state. A substantial portion of such capacitance is associated with the control terminal of the semiconductor device, e.g., a gate terminal in the FET technology, a base terminal in bipolar junction transistor technology, and/or the like, and therefore, is integral to switching capability.
For example, FIGS. 1A and 1B show illustrative FETs 2A, 2B, respectively, configured to operate as RF switches according to the prior art. As illustrated in FIG. 1A, when FET 2A is in an off state, the RF channel 6 below gate 2A is depleted. However, RF leakage occurs through edge gate 2A to channel 6 capacitance. Similarly, FIG. 1B illustrates the RF leakage for a FET 2B incorporating a more advanced, multi-gate 4A-C design.
A current approach uses Gamma or Pi-type circuits that include a combination of series and shunt-connected semiconductor devices to improve the OFF state isolation of an RF switch. However, such an approach is not suitable for many important RF switch applications. For example, a multi-channel switch is an important component of many modern microwave systems. Assume a series-shunt switch is connected into channel A of the multi-channel switch and channel A is turned off. In this case, due to capacitive coupling through the series device at high operating frequencies, the shunting device will substantially shunt the RF source and any other RF channels, which are supposed to be turned ON when channel A is turned off.